SALT LAKE CITY — Utah has the constitutional authority to define marriage, and the union of a man and a woman is the best setting to bear and rear children, attorneys for the state argue in an appeals court brief filed just before midnight Monday.

They also contend that traditional marriage furthers the state's interests in accommodating religious freedom and preserving social harmony in the state, while redefining marriage would be a recipe for social and religious strife.

"Redefining marriage as a genderless, adult-centric institution would fundamentally change Utah's child-centered meaning and purpose of marriage," according to a 120-page brief filed in the 10th Circuit Court of Appeals.

"A society can have but one understanding of marriage: It is either a uniquely man-woman institution, or it is not. Because man-woman unions are unique in their ability to produce children, maintaining the man-woman definition reinforces the child-centric view of marriage."

The filing is the state's opening volley in its appeal of U.S. District Judge Robert J. Shelby's Dec. 20 ruling that Utah's voter-approved definition of marriage violates same-sex couples' right to equal protection under the 14th Amendment.

State attorneys made some of the same arguments in previous court filings regarding Amendment 3, but newly hired lead counsel Gene Schaerr articulated them more in depth and with more supporting evidence.

But Salt Lake attorneys Brett Tolman and Paul Burke, who represented the Utah Pride Center at the U.S. Supreme Court in the federal Defense of Marriage Act case, say the state relies on "junk science" to indict gay parents and demean their families.

"The state of Utah's latest defense of Amendment 3 offers only a new candor that the state wants to consign LGBT Utahns to second-class citizenship based on the belief they make third-rate parents," they said in a statement. "The state's arguments are flawed and will ultimately fail because the U.S. Constitution neither knows nor tolerates classes among American citizenship."

"The people of Utah can have confidence that their overwhelming majority view of marriage and family has been well represented. Indeed, the state’s argument sets the new legal standard in defense of the state interest in traditional marriage and the natural family," the conservative political think tank said in a statement.

Utah Attorney General Sean Reyes also issued a statement Tuesday, saying the brief clarifies why the appeals court "should uphold the decision of Utah’s citizens to retain the marriage definition that has been in place since the state’s inception."

"The legal question at issue is not the fundamental right of same-sex couples to enter into exclusive and permanent relationships, raise children or bequeath property at their death. Utah law already gives those rights," Reyes said.

"The constitutional question is whether it is reasonable for Utah's citizens to believe that a child benefits most from being raised by his or her biological mother and father in a permanent relationship, and that such relationships should therefore be encouraged through recognition as marriages."

Reyes has support from Republican state lawmakers, whom he briefed on the case last month.

Senate President Wayne Niederhauser, R-Sandy, said it is the state’s "duty to defend our jurisdiction over the definition of marriage."

Senate Minority Caucus Manager Luz Robles, D-Salt Lake City, would not comment for Senate Democrats as a whole but noted "there are many types of families."

While voters may have approved Amendment 3 in 2004, “there have been a lot of changes (since then)," Robles said.

Shelby's court ruling came after three gay and lesbian couples challenged the state's ban on same-sex marriage in a federal lawsuit last March.

Utah appealed the decision and asked the 10th Circuit to put the ruling on hold, but the Denver-based court rejected the request. The state turned to the U.S. Supreme Court, which issued a stay on Jan. 6. Approximately 1,300 couples were married in Utah during that 17-day period.

Schaerr contends in the filing that Shelby failed to give adequate weight to Utah's reasons — which he says are supported by common sense and substantial social science — for retaining its definition of marriage. And, he says, the judge came to an erroneous conclusion based on a "novel" reading of constitutional guarantees of due process and equal protection.

"Contrary to the district court’s conclusion, there is no fundamental 14th Amendment right to a state marriage certificate allowing two people of the same sex to marry," he wrote.

Utah's marriage definition does not interfere with adults’ ability to commit to an exclusive, loving relationship with others of the same sex, or to bring children into that relationship, according to the brief.

"Instead, the laws at issue here simply encourage a familial structure that has served society for thousands of years as the ideal setting for raising children," Schaerr wrote. "Nothing in the federal Constitution prevents Utah’s citizens from making that choice."

Besides avoiding risks to religious freedom and social peace, traditional marriage advances at least three legitimate and compelling state interests, according to the brief.

Specifically, it:

Prevents further erosion of the traditional concept of marriage as being focused first and foremost on the welfare of children rather than the emotional interests of adults.

Increases the likelihood that children will be raised by their biological mothers and fathers — or at least a mother and father — in intact families.

Ensures adequate reproduction by parents willing and able to raise their children in stable homes.

The state cites numerous social science studies and reports and court cases to support its argument, including data that show birth rates decline in countries and states that have adopted "genderless" definitions of marriage.

State attorneys also maintain that religious support for defining marriage as between one man and one woman is widespread and deeply rooted in all three major Abrahamic faiths — Christianity, Judaism and Islam — and one of the other two largest world religions — Buddhism.

"Millions of Utahns who accept these traditions understand marriage and sexuality as gifts from God, designed not principally for the gratification of adults, but to provide an optimal setting for bearing and raising children," according to the brief.

Citing a group of pro-same-sex marriage law professors, the state argues that redefining marriage "could create a whole new set of problems for the religious liberty of those religious believers who cannot conscientiously participate in implementing the new regime."

Those include government being pressured to revoke churches' tax-exempt status, force religious agencies to cease adoption and foster care services, and punish religious colleges for adhering to their views on marriage in student housing, curriculum and hiring, according to the brief.

The plaintiffs' response to the state's appeal is due Feb. 25. The court has scheduled oral arguments for April 10.

Three nonprofit groups — the Center for Urban Renewal and Education; Coalition of African American Pastors USA and Frederick Douglass Foundation — jointly filed Tuesday the first what could be a dozen amicus or friend-of-the-court briefs supporting both sides in the case.

The brief argues that the Supreme Court decision in Loving v. Virginia, which overturned that state's ban on interracial marriage in 1967, is not analogous to the Utah same-sex case as Shelby asserted in his ruling.

Republican leaders in the Utah Senate also expressed their support Tuesday of the state’s latest legal arguments filed in defense of Amendment 3.